Urea is manufactured by the following method: first, ammonia (NH3) and carbon dioxide (CO2) are subjected to a reaction to produce ammonium carbamate (NH2COONH4) as represented by Formula (1), and then, ammonium carbamate is subjected to a dehydration reaction to produce urea (NH2CONH2) and water (H2O) as represented by Formula (2).2NH3+CO2→NH2COONH4  (1)NH2COONH4→NH2CONH2+H2O  (2)Both reactions are the equilibrium reaction but the reaction of Formula (1) is the exothermic reaction while the reaction of Formula (2) is the endothermic reaction. For this reason, it has been difficult to efficiently manufacture urea from the raw materials of ammonia and carbon dioxide and various devises therefore have been studied.
Patent Literature 1 has described the technique made for the purpose of reducing the amount of water to be returned to the synthesis zone. In this technique, the solution from the stripper is processed in at two stages which is conducted at medium pressure and low pressure. In this manner, the remaining unreacted substances are removed. The solution from the stripper is expanded at the expansion valve 17 and placed under reduced pressure of from 12 to 30 bar and said solution become a fluid in gas-liquid phase. The fluid is introduced into the gas-liquid separator 6, where the fluid is separated into the gas and the liquid. Of the separated gas and liquid, just the liquid is introduced into the tube side of the first high pressure condensation zone 3, which is the horizontal condensation reactor under the further reduced pressure. The liquid is discharged after being heated by the heat of condensation of the decomposed gas or the like from the stripper in the first high pressure condensation zone 3. The liquid heated by the heat of condensation is also introduced into another gas-liquid separator 7 so that gas is separated therefrom. The liquid is introduced into the contact zone 8 (corresponding to the medium-pressure decomposition column). The unreacted substances of the liquid are stripped adiabatically using the gas from the gas-liquid separator 6. The pressure is further reduced and the gas and liquid are separated from the liquid introduced into the gas-liquid separator 9. The separated liquid is introduced into the heat exchanger 10 (corresponding to low-pressure decomposition column). The gas from the gas-liquid separators is recovered, condensed, and returned to the synthesis zone (the first high pressure condensation zone 3).
Patent Literature 2 has described the method of introducing the urea synthesis solution from the stripper into the tube side of the bubble column type vertical condensation reactor, thereby heating the solution. This method enables the higher heat transfer performance than the conventional technique, and with this method, the heating can be carried out at higher temperature level (from 170 to 180° C.). For this reason, this method is featured in that the heat transfer area can be drastically reduced. However, this patent literature does not particularly mention the removal of the unreacted substances in the medium-pressure decomposing process.